1. Field of the invention
This invention relates to a device for monitoring a warp and a fabric in a power loom, in particular, but not exclusively, a loom for the weaving of wires and felts for, primarily, paper-making machines.
2. Description of the Prior Art
To facilitate an understanding of the prior art and the problems associated with such weaving, the structure of a power loom used in that connection will first be described with reference to FIG. 1 of the accompanying drawings, which is a schematic view of a power loom.
Referring to FIG. 1, the longitudinal yarns, which pass through the loom to form the warp 1, are unwound from bobbins 2 which usually have an axial length of from 2 to 5 dm. The bobbins 2 are mounted on a warp beam 3, the axial length of which determines the maximum width of the woven fabric. The width of the loom is from 10 to 20 m, but looms for a maximum width of 30 m are currently being manufactured. The yarn diameter in the case of weaving wire fabric is normally of the order of magnitude of from 0.15 to 0.35 mm. The yarn density in the warp 1 normally amounts to about 25 ends per cm.
The warp 1 passes over an auxiliary back roll 4 and a movably mounted whip roll 5, and then to a dobby, which comprises a shaft frame 6 with a heald 7. The task of the dobby is alternately to raise and lower every second warp yarn in order to form a shed 8 to make room for the transverse passage or picking of a shuttle 9 with weft yarn between the upper and lower layers of warp yarns of the shed 8. The shuttle 9 rests on a shuttle race 10 at the level of the lower layer of warp yarns of the shed 8. A reed 11, which is a yarn controlling device with a slot or groove for each warp yarn, is secured to the shuttle race 10, the shuttle race and the reed being mounted on an arm 12 which is pivotally mounted at its lower end.
After a weft yarn has been picked by the shuttle 9 between the upper and lower layers of warp yarns of the shed 8, the upper part of the arm 12 moves in the longitudinal direction of the loom, with the result that the weft yarn that has just been inserted is beaten by the reed 11 with great force against the previously inserted weft yarns. At this point the warp 1 changes into a fabric 13, which passes on over a breast beam 14 to a number of fabric beams 15, 16 and 17, and thence to a final take-up beam (not shown) for the finished woven fabric.
The feeding speed of the warp 1 and the fabric 13 during normal operation amounts to about 0.5 to 5 cm/min. The shuttle 9 is picked with great force, and the number of picks per minute depends on the weaving width of the loom. The weft density for one and the same weaving speed can be selected by means of different gear ratios between the central driving means and the driving means of the fabric beams. Normally the weft density is from 15 to 50 yarns/cm of fabric.
Looms of the above-described type are driven in one of two ways. The first and older of the two ways involves the provision of a main drive which drives the fabric beams 15, 16 and 17, so that the warp 1 and the fabric 13 are pulled forwardly through the loom. From this main drive, the dobby, the arm 12 with the shuttle race 10 and the reed 11, the picking mechanism for the shuttle 9 and other mechanisms of the loom are activated by means of different auxiliary mechanisms. The warp beam 3 is mechanically braked.
The second way of driving such looms, which is used when there are greater demands on the shape of the fabric, is similar to the first way, but involves the replacement of the mechanical braking device of the warp beam 3 by an electrical braking device. This electrical braking device comprises a d.c. drive means which is part of a warp tension control system. Since the present invention is concerned primarily with looms driven in this second way, the driving system will now be described in more detail with reference to FIG. 2 of the accompanying drawings, which is a schematic diagram of the driving system.
Referring to FIG. 2, the means which carries out the tension control includes a conventional speed control arrangement designated nref, a speed regulator 18, a static convertor 19, a motor 20 with a gear unit 21, and a tachometer 22 for feedback. The gear ratio of the gear unit 21 is very high, which means that the motor 20 with the gear unit 21 is self-locking. The tension control is superordinate to the speed control. The tension is measured by a load cell 23 situated at the breast beam 14 (see FIG. 1). The desired tension is set by a tension reference Tref, and the output of the tension regulator 24 is supplied to the speed regulator 18 as an additional reference.
The circumferential speed of the warp beam 3 must correspond substantially to the speed at which the driving means of the fabric beams draws the fabric forwards. As the yarn is unwound from the bobbins 2, the rotational speed of the warp beam 3 must therefore be adjusted to adapt to the feeding speed of the fabric. In principle, this adjustment could be performed by the addition of the tension control fault to the speed reference. However, it is desirable for the tension regulator to operate with zero mean fault, and, moreover, it is highly desirable to be able to start up the loom with the correct speed on the warp beam after a stoppage. The tension fault signal is therefore utilized for continuously increasing or decreasing the speed reference via a motor operating device 25 and a readjustment device 26, so that the tension fault regulator may operate around zero.
The prior art regarding the control of the warp, the fabric, the weft density, etc., at present involves a sporadic inspection of the fabric with a magnifier and a ruler or with a microscope.
In a perfect fabric there is the same distance between the yarns, or possibly a regularly recurring pattern. Thus, a constant weft density is aimed at. If the fabric is an endless wire for a paper-making machine, disturbances and irregularities in the weft density may affect the quality of the paper made on the finished wire, with respect to marks as well as the thickness of paper produced. Disturbances of the weft density may also initiate wire breakage with considerable economic consequences.
For technical reasons, a direct control of the weft density has not been possible in the past, among other things because of the unavailability of measuring equipment for sensing the weft density. A great improvement in the quality and uniformity of the fabric was obtained with the introduction of tension control, as shown in FIG. 2, as compared with the corresponding control in connection with mechanical braking of the warp beam. However, the tension control still only constitutes an indirect control of the weft density.
A power loom of the kind described above is a relatively compact structure, which implies that a continuous monitoring of the fabric during operation is at present not feasible in practice. If an irregular weft density or other fault occurs, it may be difficult to discover this quickly enough. The consequence of this is that quite a long piece of fabric can be produced before the irregularity can be observed. The fabric feed must then be stopped, and the incorrect weft yarns must be removed. The warp and the fabric must then be moved backwards so that the new edge of the fabric assumes the correct position for insertion of a new weft yarn.
In the case of yarn breakage in connection with picking of the shuttle through the shed, driving of the loom is stopped. When the weaving is restarted after the broken yarn has been removed and a new shuttle has been prepared, a variation in weft density often occurs in the fabric due to the position of the edge of the fabric (weft edge) having been changed during the interruption. This is due to the fact that the yarn material is elastic and ductile and to the fact that, when the loom remains stationary under tension for a period of time, the warp is stretched and to a certain extent the warp is straightened out between the weft yarns in the fabric. Therefore, the weft edge is displaced and this results in an undesirable variation in weft density in the fabric.
The movement of the weft edge when the loom has stopped is at present a considerable problem, since accurate determination of the position during a stoppage and immediately prior to restart cannot be performed using known measuring techniques applied to such looms.
As will be clear from the above description of the prior art and the problems connected therewith, a measuring device which is able to indicate with sufficient accuracy the position of the weft edge and the weft density could be utilized for correctly positioning the weft edge after an interruption of the driving of the loom, for monitoring the fabric, and also for inspection and control of the weft density.